Erosive synovitis can be induced experimentally in rats by intraperitoneal injection of bacterial cell walls from group A streptococci. Genetic factors determine susceptibility to development of disease. Susceptible inbred rat strains develop marked mononuclear cell inflammation, most evident in peripheral joints. Resistant inbred strains show little evidence of pathology. Susceptibility is a dominant trait, but is determined by multiple genes. Immunologic characterization has revealed marked differences in response to cell walls between susceptible and resistant strains.